title: Low cytosine triphosphate synthase 2 expression renders resistance to 5-fluorouracil in colorectal cancer.
abstract: Understanding the determinants of resistance of 5-fluorouracil (5FU) is of significant value to optimising administration of the drug, and introducing novel agents and treatment strategies. Here, the expression of 92 genes involved in 5FU transport, metabolism, co-factor (folate) metabolism and downstream effects was measured by real-time PCR low density arrays in 14 patient-derived colorectal cancer xenografts characterised for 5FU resistance. Candidate gene function was tested by siRNA and uridine modulation, and immunoblotting, apoptosis and cell cycle analysis. Predictive significance was tested by immunohistochemistry of tumours from 125 stage III colorectal cancer patients treated with and without 5FU. Of 8 genes significantly differentially expressed between 5FU sensitive and resistant xenograft tumours, CTPS2 was the gene with the highest probability of differential expression (p=0.008). Reduction of CTPS2 expression by siRNA increased the resistance of colorectal cancer cell lines DLD1 and LS174T to 5FU and its analogue, FUDR. CTPS2 siRNA significantly reduced cell S-phase accumulation and apoptosis following 5FU treatment. Exposure of cells to uridine, a precursor to the CTPS2 substrate uridine triphosphate, also increased 5FU resistance. Patients with low CTPS2 did not gain a survival benefit from 5FU treatment (p=0.072), while those with high expression did (p=0.003). Low CTPS2 expression may be a rationally-based determinant of 5FU resistance.
authors.affiliation: Cancer Science Institute of Singapore, Center for Life Sciences, National University of Singapore, Singapore.
authors.forename: Wen Lee
authors.initials: WL
authors.lastname: Tan
authors.forename: Bhaskar
authors.initials: B
authors.lastname: Bhattacharya
authors.forename: Marie
authors.initials: M
authors.lastname: Loh
authors.forename: Indirikumar
authors.initials: I
authors.lastname: Balasubramanian
authors.forename: Mohamed
authors.initials: M
authors.lastname: Akram
authors.forename: Difeng
authors.initials: D
authors.lastname: Dong
authors.forename: Limsoon
authors.initials: L
authors.lastname: Wong
authors.forename: Bhavin
authors.initials: B
authors.lastname: Thakkar
authors.forename: Manuel
authors.initials: M
authors.lastname: Salto-Tellez
authors.forename: Ross A
authors.initials: RA
authors.lastname: Soo
authors.forename: Iduna
authors.initials: I
authors.lastname: Fichtner
authors.forename: Barry
authors.initials: B
authors.lastname: Iacopetta
authors.forename: Richie
authors.initials: R
authors.lastname: Soong
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics; D015179:Colorectal Neoplasms / Q000188:drug therapy* / Q000235:genetics / Q000473:pathology; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D005472:Fluorouracil / Q000627:therapeutic use*; D006801:Humans; D051379:Mice; D011755:Pyrophosphatases / Q000235:genetics*; D034741:RNA, Small Interfering / Q000235:genetics; D014529:Uridine / Q000378:metabolism; D023041:Xenograft Model Antitumor Assays
chemical_list: D034741:RNA, Small Interfering; C019950:CTPase; D011755:Pyrophosphatases; D005472:Fluorouracil; D014529:Uridine